1. Technical Field of the Invention
The present invention relates generally to an automatic transmission control system for automotive vehicles, and more particularly to such a system working to monitor a failure in operation of a shift lever position sensor and/or to ensure running of the vehicle if the shift lever position sensor is failing.
2. Background Art
Japanese Patent First Publication No. 2003-294134 discloses a shift lever position sensor implemented by an inhibitor switch equipped with four contacts S1, S2, S3, and S4 which are turned on or off following the motion of a gear shift lever manually shifted by a vehicle driver. Patterns of combinations of the on- or off-states of the contacts S1 to S4 are designed to be different among a parking (a, a reverse (R), a neutral (N), and a drive (D) position of the gear shift lever. Specifically, each of the patterns represents one of the P, R, N, and D position of the gear shift lever.
The above type of inhibitor switch has the problem in that if a failure such as a wire disconnection or a short circuit occurs in any of the contacts S1 to S4, an error signal is outputted.
In order to avoid an error in determining the N and P positions of the gear shift lever arising from the failure of the inhibitor switch, the inhibitor switch, as taught in the above publication, is so designed that if any one of the contacts S1 to S4 outputs an error signal when the gear shift lever is placed in the N position, the pattern of combination of the on- and off-sates of the switches S1 to S4 in the N position does not match that in the P position and that it may be determined the inhibitor switch is failing when a pattern of a combination of outputs from the switches S1 to S4 does not match any of those as representing the P, R, N, and D position of the gear shift lever correctly.
The above type of inhibitor switch, however, has the drawback in that if a pattern of a combination of outputs of the switches S1 to S4 matches any one of those as representing the P, R, N, and D position of the gear shift lever correctly, a difficulty is encountered in determining the presence of the failure of the inhibitor switch, and the gear shift lever may be determined in error as being placed in a position other than the correct one. This may cause the gear shift lever to be determined in error to be placed in a position other than the D position, even though the gear shift lever is actually in the D position, thus resulting in the impossibility of operating the automatic transmission to run the vehicle.